PROJECT SUMMARY Secretory diarrhea is a major cause of morbidity and mortality in children resulting from the overstimulation of chloride (Cl-) channels in intestinal epithelial cells (IECs) that leads to overwhelming fluid secretion and life- threatening dehydration. These Cl- channels are regulated by cyclic nucleotides (cAMP and cGMP) and calcium (Ca2+) signaling pathways, but there are gaps in knowledge in how pathogens, such as rotavirus (RV), cause diarrhea through elevation of cytosolic Ca2+ ([Ca2+]c). RV persistently activates stromal interaction molecule 1 (STIM1) in infected cells, which in turn activates processes for extracellular Ca2+ entry at endoplasmic reticulum-plasma membrane (ER-PM) junctions. Activated STIM1 causes formation of ER-PM junction protein complexes that regulate Ca2+ and cAMP signaling and potentially the activation of Ca2+- activated and cAMP-activated chloride channels. Understanding the identity and activation of Cl- channels in RV infection is significant for advancing scientific knowledge of fluid secretion in the GI tract as well as future development of anti-secretory therapies to treat infectious and functional diarrhea. Thus the objective of this research is to identify the Cl- channels activated during RV infection and the regulatory role of activated STIM1 in ER-PM junctional complexes in their activation. I hypothesize that persistent activation of STIM1 stimulates the activation of Ca2+- and cAMP-dependent Cl- channels for RV-induced diarrhea. Using human intestinal enteroids (HIEs) genetically modified with biosensors as a model of IECs and a mouse model of RV diarrhea, I propose [1] to identify the Cl- channels activated during RV infection and [2] to determine the dynamic composition of ER-PM junctions with STIM1 activation in RV-infected IECs. Results from the proposed project will significantly advance our current understanding of molecular mechanisms of Cl- secretion in IECs, which can be extended to study other enteric viruses and bacterial pathogens. These findings will provide new insights into the regulatory roles of ER-PM junctions in the specialization of cell functions.